SAD is a “condition of seasonal depression characterised by low mood, feelings of sadness, anxiety and fatigue”.

She pointed to research by the University of Georgia, who found lack of vitamin D is an important factor.

Jenny explained: “Researchers at the University of Georgia who conducted a meta-analysis of more than 100 articles believe the role of vitamin D in the synthesis of serotonin and dopamine may be one important contributory factor.”

READ MORE: Vitamin D deficiency symptoms: Six signs you should never ignore

Vitamin D deficiency symptoms can be difficult to spot, but one of the main symptoms can be low mood.

If we aren’t exposed to enough sunshine, there will be a lack of production of the so-called happy hormone, or serotonin.

Serotonin is a neurotransmitter that regulates mood.

This is why vitamin D deficiency can lead to SAD, which is also known as “winter depression”.

READ MORE: Vitamin D toxicity: Can you overdose on vitamin D? This is how much you should get

Vitamin D deficiency: The main source of the vitamin is sunlight (Image: GETTY)

Lack of sunlight can affect the production of serotonin, but also the production of melatonin – the hormone which makes you feel sleepy.

For those who have SAD, the body may produce it in higher levels than normal.

Other symptoms of SAD, in addition to low mood, includes loss of interest in normal activities, irritability, feelings of despair, guilt and worthlessness and sleeping longer than usual.

The NHS recommends to consider seeing your GP if you think you might have SAD and you’re struggling to cope.